An inflatable tubular beam, also known as an air beam, is a structural support element having a pre-shaped structure, e.g., a cylindrical tube, of flexible material which is inflated to develop its rigidity. Air beams are particularly useful in situations where light weight and/or compact storage capability of the uninflated element are desired.
Inflated air beams can take various shapes and forms. Arched air beams are used, inter alia, in rapidly deployable shelters. Due to the light weight and compactness of the inflatable beams, such shelters are more conveniently transported, more quickly erected, and require less labor than conventional rigid structures.
Inflated, arched, or curved beams are also used for spars in deployable, arched, wings, such as parafoils and paragliders. The advantages of using a plurality of internally-pressurized partial-structures to support the wing, compared to ram-air inflation of the entire wing, are increased performance and improved safety.
It is known in the prior art to produce an inflatable curved or arched tubular beam or air beam by providing a gas-impermeable elastomeric or polymer film tubular lining or air bladder inside a fiber reinforced outer sleeving, such as a braided sleeve. For this purpose, it has been suggested to produce the sleeve by braiding the fibers directly onto a curved mandrel duplicating the desired shape of the final part. For each given curved or arched air beam, a customized mandrel having a specific size and shape is required.
It is also known in the art to produce a tubular air beams by braiding a fiber sleeve directly over an air bladder or elastomeric liner or tube of thin elastomeric film. To fix the braided structure to the air bladder of elastomeric liner, an adhesive may be applied to the surface of the air bladder or elastomeric liner prior to applying the braid.
To produce an air beam of sufficient strength using the methods described above, the fixed mandrel or air bladder is over-braided with a multi-layer structure, including a number of structural layers of fibers. These layers may be at least partially intertwined. In this multi-layered structure, each structural layer includes both axial and bias fibers.
U.S. Pat. No. 5,421,128, the disclosure of which is incorporated herein by reference, describes a method of producing a curved air beam by braiding a triaxial-braid sleeve over an elastomeric liner. According to the method of U.S. Pat. No. 5,421,128, the axial fibers of the triaxial braid are present only over a portion of the tube circumference, e.g., over less than 60 degrees of the 360-degree tube circumference, while the remainder of the circumference includes only non-axial bias fibers. In this configuration, a portion of the tube is constrained to be substantially inexpansible whereby, upon inflation, the tube curves with the constrained axial fiber portion defining the inside curve of the curved beam. Weak portions of the curved structure, e.g., the outside curve of the beam, may be reinforced by reinforcing means such as tape attached to the outside of the inflated tube. Instead of using an air bladder or pre-formed elastomeric lining, the braided fibers may be impregnated with an elastomeric solution that forms a gas barrier after curing. The fibers may be optionally impregnated with a solution that forms a hard outer surface after curing.
There are number of deficiencies to the air beam described in U.S. Pat. No. 5,421,128. One deficiency of this air beam is that the curvature of the beam is produced, upon inflation, by an equilibrium between constrained bias fibers (i.e., bias fibers which are constrained by axial fibers) and unconstrained bias fibers. Because of this requirement, the bias fiber orientation is dictated by the desired curvature of the beam and, therefore, the bias fiber orientation cannot be optimized based on the inflation requirements and/or the service loads for which the beam is designed. It should be noted that the optimum fiber orientation for a given beam curvature is generally different from the optimum fiber orientation for given service loads and/or inflation requirements and, therefore, additional reinforcement materials and/or processing steps are generally required to adapt the air beam for a given use.
Further, because the curvature of the air beam described in U.S. Pat. No. 5,421,128 is controlled by the location of axial fibers only on part of the beams circumference, additional materials and/or steps are required to provide a desired reinforcement of the non axially-reinforced portion.